Method of remediating cyanide-contaminated soil

ABSTRACT

Provided is a method of remediating cyanide-contaminated soil. The method is provided to remediate soil contaminated with cyanide and treat the cyanide, which includes collecting the soil contaminated with first cyanide in a solid state and second cyanide in a gaseous or dissolved state, dissociating cyanide by mixing the soil with an alkali washing solution, dissolving the first cyanide in a solid state in the washing solution, and transferring the second cyanide in a dissolved state dissociated from the soil to the washing solution, dissociating the soil from the washing solution, precipitating the first cyanide in a solid state by acidifying the washing solution containing the cyanide, and performing post-treatment on the first cyanide after the first cyanide precipitated in a solid state is dissociated from the washing solution.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to and the benefit of Korean PatentApplication No. 2010-0003804, filed 15, Jan. 2010, the disclosure ofwhich is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of remediating contaminatedsoil, and more particularly, to a method of remediatingcyanide-contaminated soil in which cyanide-contaminated soil isremediated and cyanide in the soil is treated.

2. Description of Related Art

It is reported that cyanide is used in various applications such asmining, surface treatment for metal, production of aluminum and iron,and manufacture of agricultural pesticides, and as of 2001, 950000 tonsof cyanide has been produced worldwide.

Cyanide has a bad impact on humans and the ecosystem. For example, whenKCN or NaCN enters a human body through an oral route, HCN is releasedby gastric acid, and when HCN is absorbed into a human body such as amucous membrane or lung, it inhibits oxygen delivery driven byhemoglobin. As a result, if more than a predetermined amount of cyanideis absorbed in the human body, difficulty in breathing or respiratoryparalysis could result. As described above, cyanide has a bad impact onhumans and the ecosystem, and thus an effluent standard for waste wateris set lower than 0.01 mg/L according to the water quality standards,and a content of cyanide in soil is also restricted to less than 2 mg/kgby the soil environment conservation act.

However, a considerably wide range of soil is contaminated due to theleakage of cyanide from plating plants or metal treatment plants, or thedisposal of cyanide-containing waste exceeding the standard. Thus,remediation of the cyanide-contaminated soil is required.

Purification of the cyanide-contaminated soil is performed using variousprocesses such as room temperature oxidation, high temperaturedegradation and biological degradation. The biological degradation ishighly economical since it is a low-cost process, but takes a longperiod of time. The high temperature degradation is applied for soilcontaminated with cyanide which has a high concentration and lowsolubility. This process is efficient in treating the contaminated soilbut should be performed at high temperature, and thus requires highcost. For this reason, it is not good for treating soil in a largescale. The room temperature oxidation is a lower cost process than thehigh temperature degradation, which is suitable for soil contaminatedwith cyanide having a low concentration that is easily degradable.However, it is not suitable for high concentration cyanide orsolid-state cyanide.

In other words, the room temperature oxidation is a technique ofinjecting an oxidizing agent such as hydrogen peroxide or ozone intocontaminated soil to degrade cyanide. It is known that the oxidizingagent such as hydrogen peroxide or ozone has a very low degradationcapability to solid cyanide having a low solubility and strong aciddissociable (SAD) CN such as Fe(CN)₆ ⁴⁻. However, there are variouslimitations to remediating cyanide-contaminated soil using the roomtemperature oxidation since most of cyanide included in soil is a solidSDA-type.

There is also a problem in treating easily-degradable low-concentrationcyanide. That is, an oxidizing agent injected into soil is consumed todegrade cyanide, and a significant amount of the oxidizing agent is alsoconsumed by organic materials, manganese oxide and sulfide mineralscontained in the soil. For this reason, to obtain a desirable result, alarger amount of an oxidizing agent than a substantially required amountof the oxidizing agent should be injected into contaminated soil.

Therefore, development and application of a technique capable ofremediating cyanide-contaminated soil in a short time at a low cost atroom temperature are needed.

SUMMARY OF THE INVENTION

The present invention is directed to a method of remediatingcyanide-contaminated soil capable of economically treating cyanideincluded in the soil at room temperature, and particularly, effectivelyremoving solid cyanide having a low solubility from the soil.

One aspect of the present invention provides a method of remediatingcyanide-contaminated soil, which is employed to remediate soilcontaminated with cyanide and treating the cyanide. The method includes:collecting the soil contaminated with first cyanide in a solid state andsecond cyanide in a gaseous or dissolved state; dissociating cyanide bymixing the soil with an alkali washing solution, dissolving the firstcyanide in a solid state in the washing solution, and transferringsecond cyanide in a dissolved state dissociated from the soil to thewashing solution; dissociating the soil from the washing solution;precipitating the first cyanide in a solid state by acidifying thewashing solution containing the cyanide; and performing post-treatmenton the first cyanide after the first cyanide precipitated in a solidstate is dissociated from the washing solution.

According to the present invention, an oxidizing agent may further beadded to the washing solution dissociated from the solid to oxidize thesecond cyanide and acidify the washing solution, thereby removing thesecond cyanide.

In addition, according to the present invention, during the dissociationof the cyanide, the washing solution may have a pH of 9 to 12, andpreferably a pH of 11 to 12.

According to the present invention, the washing solution may be aphosphate solution such as pyrophosphate (P₂O₇ ⁴⁻), having aconcentration of 30 to 50 mM.

According to the present invention, during the removal of the firstcyanide, a sulfuric acid solution may be input to oxidize the washingsolution, and an ionic acid may be added to the washing solution.

According to the present invention, after the removal of the remainingcyanide, the washing solution may be adjusted in pH and concentration tobe reused in the dissociation of the cyanide.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the presentinvention will become more apparent to those of ordinary skill in theart by describing in detail preferred embodiments thereof with referenceto the attached drawings in which:

FIG. 1 is a schematic flowchart illustrating a method of remediatingcyanide-contaminated soil according to an exemplary embodiment of thepresent invention;

FIG. 2 is a table showing types of cyanide which can be present in soil;

FIG. 3 is a graph showing a total content of cyanide erupted from soilaccording to time spent treating various types of a phosphate solutionduring a process of dissociating cyanide;

FIG. 4 is a graph showing a total content of cyanide erupted from soilaccording to the concentrations of phosphate solutions having variouspHs during a process of dissociating cyanide; and

FIGS. 5 and 6 are tables showing a result of an experiment on a methodof remediating cyanide-contaminated soil according to an exemplaryembodiment of the present invention, of which FIG. 5 shows a result ofdissociation of cyanide using a washing solution, and FIG. 6 shows aresult of precipitation of first cyanide, removal of second cyanide andremoval of remaining cyanide.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, the present invention will be described with reference tothe accompanying drawings in detail. This invention may, however, beembodied in different forms and should not be construed as limited tothe embodiments set forth herein. Rather, these embodiments are providedso that this disclosure will be thorough and complete, and will fullyconvey the scope of the invention to those skilled in the art. Likenumbers refer to like elements throughout the specification. In thedrawings, the thickness of layers and regions are exaggerated forclarity.

A method of remediating cyanide-contaminated soil according to anexemplary embodiment of the present invention will be described infurther detail with reference to the accompanying drawings.

FIG. 1 is a schematic flowchart illustrating a method of remediatingcyanide-contaminated soil according to an exemplary embodiment of thepresent invention, and FIG. 2 is a table showing types of cyanide whichcan be present in soil.

Referring to FIG. 1, a method 100 of remediating cyanide-contaminatedsoil according to an exemplary embodiment of the present inventionincludes processes of dissociating cyanide 20, dissociating a solid froma liquid 30, precipitating first cyanide 41, removing second cyanide 42,and removing remaining cyanide 50.

To perform the method 100 of remediating cyanide-contaminated soilaccording to the exemplary embodiment of the present invention, first, aprocess of collecting soil 10 is performed. In the process of collectingsoil 10, soil contaminated with cyanide is collected using an excavator.Cyanide-contaminated soil is usually found on the periphery of miningplants, plating plants, or plants producing aluminum or iron.

Referring to FIG. 2, cyanide is present in soil in various forms. Thatis, cyanide may be present in soil in a solid, dissolved or gaseousstate, and classified into weak acid dissociable (WAD) cyanide, strongacid dissociable (SAD) cyanide and free cyanide according to properties.

That is, the free cyanide is present in a gaseous state or as an ionizedCN— in an aqueous solution, and gaseous HCN is very easily soluble inwater. The WAD cyanide is formed by bonding cyanide to a metal such asCu(CN)₃ ²⁻ or Zn(CN)₄ ²⁻. The WAD cyanide has a low bonding strengthbetween the metal and the cyanide, and thus is easily soluble in anenvironment having a pH of 4 to 6.

Therefore, the WAD cyanide can be easily changed from a solid state to adissolved state depending on the surrounding environment. The SADcyanide is formed by bonding cyanide to a heavy metal such as iron,cobalt or platinum like Fe(CN)₆ ⁴⁻ and Au(CN)₂ ⁻. Compared to the WADcyanide, the SAD cyanide has a very strong bonding strength between thecyanide and the metal, and thus has a very low solubility to an acid andis soluble only in an acid having a pH of about less than 2. Therefore,in neutral and acidic environments, the SAD cyanide is stably present ina solid state.

That is, the WAD cyanide and the free cyanide are usually present ingaseous and dissolved states, and the SAD cyanide is usually present ina solid state. Such a difference is caused by its solubilitycharacteristic.

Hereinafter, for convenience of description, the solid cyanide isreferred to as first cyanide, and the gaseous or dissolved cyanide isreferred to as second cyanide. Here, the first cyanide is mostly SADcyanide, but may be WAD cyanide present in a solid state depending onenvironment conditions. Further, the second cyanide is mostly WADcyanide or the free cyanide, but sometimes may be SAD cyanide dependingon environment conditions.

As described above, while the cyanide can be present as the firstcyanide in a solid state or the second cyanide in the dissolved andgaseous state in soil, in the soil, most of the cyanide is substantiallypresent as the first cyanide in the solid state, and only a trace ofcyanide is present as the second cyanide. Accordingly, during theprocess of collecting soil 10, the excavated soil contains a far largeramount of the first cyanide in the solid state than the second cyanide.Particularly, among the first cyanide, an iron-cyanide compound such asFe₄[Fe(CN)₆]₃, Fe₃[Fe(CN)₆]₂, Fe[Fe(CN)₆], or Fe₂[Fe(CN)₆] is frequentlyfound, which are known as materials for adjusting a cyanideconcentration of soil pore water.

After the soil is collected, the process of dissociating cyanide 20 fromthe soil is performed. During the process of dissociating cyanide 20,first cyanide and second cyanide are dissociated from the soil using awashing solution. Here, as described above, the solid-state firstcyanide has a very low solubility in an acidic or neutral environment,but as shown in Formulae (1) and (2), has a quite high solubility to analkali solution.Fe₄[Fe(CN)₆]_(3(s))+12H₂O=4Fe(OH)_(3(s))+3Fe(CN)₆ ³⁻+3e ⁻+12H⁺  Formula(1)3Fe₄[Fe(CN)₆]_(3(s))+32H₂O

4Fe(OH)_(8(s))+5Fe(CN)₆ ⁴⁻+32H⁺  Formula (2)

When the first cyanide meets an alkali washing solution, the firstcyanide is dissociated into solid iron hydride and ionized iron cyanide.The ionized iron cyanide is dissolved in the washing solution.

Meanwhile, the second cyanide present in the dissolved state in the soilis adsorbed to a surface of manganese oxide, iron oxide, or a soilorganic material by an electrical strength. The iron oxide, themanganese oxide, or the soil organic material is changed in surfacecharge according to peripheral pH. That is, it exhibits a positivecharge at a lower pH than the point of zero charge (PZC), and a negativecharge at a higher pH than the PZC. For example, since the surfacecharges are 0 when the iron oxide has a pH of about 7 to 9, themanganese oxide has a pH of 3 to 7, and the soil organic material has apH of 3, in the environment having a pH of 9 or more, the surfacecharges of all of these materials exhibit negative charges. As a result,in the acidic or neutral environment having low pH, the iron oxide, themanganese oxide or the soil organic material exhibiting a positivecharge on its surface is mixed with an alkali washing solution, and asurrounding environment is changed into alkali (particularly, a pH of 9or more) so that its surface also exhibits a negative charge.

Under the acidic condition, the iron oxide, the manganese oxide or thesoil organic material exhibits a positive charge on its surface, andthus can be electrically adsorbed to a cyanide anion. However, as thesurrounding environment is changed into an alkali condition, anelectrical bonding strength is lost, so that the cyanide ions aredissociated from the iron oxide, etc. and then transferred to a washingsolution.

That is, when an alkali washing solution is mixed with thecyanide-contaminated soil, the solid-state first cyanide is dissolved inthe washing solution, and the second cyanide electrically adsorbed tothe iron oxide, etc. loses an electrical adsorption strength, resultingin being dissociated from the iron oxide, etc., and then is transferredto the washing solution. The iron cyanide erupted as ions from the firstcyanide may also be adsorbed to the iron oxide, etc. in an instant, butloses an electrical adsorption strength, like the second cyanide, sothat it is soluble in the washing solution.

Meanwhile, a trace of free cyanide is present in the soil. The freecyanide has a dominant form of HCN, which is very toxic, at a pH of lessthan 9.24, and a dominant form of CN⁻ at a pH of more than 9.24. Here,high-toxicity HCN has strong volatility, so that it can be turned intoHCN gas. For this reason, an environment of treatingcyanide-contaminated soil can be dangerous. Thus, the process ofdissociating cyanide 20 may be performed in the environment having a pHof 9 or more, that is, in the environment in which the pH of the washingsolution may be 9 or more. Particularly, at a pH of 11 or more, the freecyanide is usually present as CN⁻, and thus the stability of thetreatment environment can be ensured.

To sum up, during the process of dissociating cyanide 20, to dissolvethe first cyanide, the washing solution may be alkali, to dissociate thecyanide ions electrically adsorbed to the iron oxide, etc., the washingsolution may have a pH of 9 or more, and to ensure the stability of theworking environment, the washing solution may have a pH of 11 or more.As a result, to satisfy all of the conditions, an alkali solution havinga pH of 11 or more may be used as the washing solution. The mostimportant thing in this process is dissolution of the solid-state firstcyanide. Thus, the necessary condition is that the washing solution bean alkali solution, preferably having a pH of 9 or more, and morepreferably having a pH of 11 or more. When the pH of the washingsolution is more than 12, compared to the condition having a pH of 11,it results in an increase in processing cost, with no significantimprovement of solubility of the first cyanide and the workingenvironment.

In the exemplary embodiment, as an alkali washing solution, a phosphatesolution having a pH of 11 or more is used. That is, since phosphateions in the phosphate solution are strongly bonded to metal ions,thereby forming a soluble complex salt, the soluble complex salt iseasily bonded to the iron oxide, the manganese oxide or the soil organicmaterial. The phosphate ions are bonded to the iron oxide, etc., theresulting product exhibits a negative charge as a whole, and thus thedissociation of the cyanide ions electrically adsorbed to the ironoxide, etc. can be accelerated. More specifically, the exemplaryembodiment uses sodium-pyrophosphate (Na₄P₂O₇.nH₂O), which, compared toorthophosphate or hexametaphosphate, includes many phosphate ions, andcan further stimulate the dissociation of the cyanide ions.

The present inventor tested whether cyanide in soil is easily erupted ornot when sodium phosphate was used as a washing solution.

That is, a phosphate solution having a phosphate concentration of 0 to100 mM was prepared using Na-orthophosphate, Na-hexametaphosphate, orNa-pyrophosphate, and adjusted to pHs of 10, 11, and 12 using 0.1N NaOHor 0.1N HCl. Next, 4 g of cyanide-contaminated soil reacted with 40 mlof 50 mM phosphate solution having a pH of 10 for 24 hours, and a totalamount of erupted cyanide according to reaction time was estimated. Theresult is shown in FIG. 3.

4 g of cyanide-contaminated soil reacted with 40 ml of 0 to 100 mMphosphate solution having a pH of 10 to 12 for 24 hours, and theconcentration and pH of cyanide were measured. The result is shown inFIG. 4. The concentration of cyanide was measured by a method accordingto the American Society for Experimenting Materials (ASTM).

Referring to FIGS. 3 and 4, compared to the Na-orthophosphate andNa-hexametaphosphate solutions, much cyanide was erupted from theNa-pyrophosphate. Particularly, from the Na-pyrophosphate, the amount oferupted cyanide was drastically increased for up to 100 minutes from thebeginning of the reaction, but after 100 minutes, gently increased.

It is confirmed that 90% or more of the total amount of cyanide eruptedafter the 24-hour reaction was SAD cyanide, 10% or less was WAD cyanide,and a trace amount was free cyanide. The contents of the erupted cyanidedescribed above are almost the same as the contents of cyanide in soilby types. In addition, through the eruption experiment, it can beconfirmed that almost all cyanide in soil was erupted. The cyanideerupted after the 24-hour reaction was increased as the concentrationand pH of phosphate were increased.

When the pH of the phosphate solution was 12 and the concentration ofphosphoric acid was increased up to 30 mM, the eruption amount ofcyanide was drastically increased, but when the pH of the phosphatesolution was 10 or 11, only after the concentration of phosphoric acidwas increased up to 50 mM, did the eruption amount of the cyanidedrastically increase. Therefore, when the concentration of phosphoricacid is optimized in the range of 30 to 50 mM by pH of the solution,cyanide can be economically erupted.

As described above, the process of dissociating cyanide 20 uses analkali washing solution, and thus only cyanide is transferred to thewashing solution, thereby dissociating the cyanide from the soil.

Afterwards, the washing solution may be dissociated from the soil toreuse the soil. That is, during the process of dissociating a solid froma liquid 30, a solid (soil) and a liquid (washing solution) aredissociated from each other by a known centrifugal separator using adifference in specific gravity. After the dissociation of the solid fromthe liquid, almost no cyanide is found in the soil, so that the soil canbe reused to remediate. However, the washing solution containing thecyanide is treated through subsequent processes to be described below.

After the process of dissociating a solid from a liquid 30, firstcyanide and second cyanide are removed from the washing solution. Morespecifically, the first cyanide and the second cyanide are removedthrough the processes of precipitating the first cyanide 41 and removingthe second cyanide 42. The processes of precipitating the first cyanide41 and removing the second cyanide 42 are accomplished by inputting anacid solution to the washing solution to acidify. Thus, the processes ofprecipitating the first cyanide 41 and removing the second cyanide 42are not performed either sequentially or separately. That is, to performthe process of removing the second cyanide 42, first, an oxidizing agentis merely input to the washing solution to oxidize the WAD cyanide andthe free cyanide, but the precipitation of the first cyanide and theremoval of the second cyanide are simultaneously performed, which willbe described in further detail below.

When an oxidizing agent such as hydrogen peroxide or ozone is input tothe solid-liquid dissociated washing solution, the free cyanide and theWAD cyanide in the washing solution are oxidized as a type of CNO⁻through the reactions of Formulae (3) and (4).CN⁻+H₂O₂--->CNO⁻+H₂O  Formula (3)M(CN)₄ ²⁻+4H₂O₂+20H⁻--->4CNO⁻+4H₂O+M(OH)_(2(s))  Formula (4)

Here, CN⁻ of Formula (3) is free cyanide, M of Formula (4) is a metalsuch as copper or zinc, and M(CN)₄ ²⁻ is WAD cyanide.

The oxidation reactions shown in Formulae (3) and (4) are easilyperformed under an alkali condition, and thus the free cyanide and theWAD cyanide in the alkali washing solution having a pH of 11 to 12 asdescribed in the exemplary embodiment are very rapidly oxidized.

Meanwhile, another reason for oxidizing the free cyanide by inputtingthe oxidizing agent is to inhibit generation of high-toxicity HCN gas.That is, during the process of precipitating the first cyanide 41, whichwill be described below, an acid solution will be input to the washingsolution. Here, if the free cyanide is not previously oxidized using theoxidizing agent, the free cyanide can react with the acid solution,thereby forming HCN having high toxicity. The HCN has strong volatilityand thus can be turned into HCN gas, which may be harmful to humans. Forthis reason, the production of HCN will be inhibited using the oxidizingagent.

As described above, after the second cyanide is oxidized, an acidsolution is input to the washing solution, thereby completing theprocess of precipitating the first cyanide 41, and then the process ofremoving the second cyanide 42 is sequentially performed.

That is, the first cyanide (most of the SAD cyanide and some WADcyanide) present in a solid state in soil is dissolved into an ionicstate in the washing solution under the basic environment through theprocess of dissociating cyanide 20. If the washing solution returns toan acidic condition, the ionic first cyanide is precipitated again intoa solid state through inverse reactions of those shown in Formulae (1)and (2). Since the first cyanide has a low solubility under an acidiccondition, it is not dissolved again in a washing solution andmaintained as a solid state.

In addition, the CNO⁻produced through the reactions shown in Formulae(3) and (4) is dissolved into ammonia and carbon dioxide under theacidic condition as shown in Formula (5).CNO⁻+2H₂O >NH₃+CO₂+OH⁻  Formula (5)

The reaction of Formula (5) is actively performed as pH is decreased. Inthe exemplary embodiment, when a sulfuric acid solution is input to awashing solution to decrease pH, thereby precipitating the firstcyanide, the second cyanide is finally removed. In the exemplaryembodiment, sulfuric acid, compared to hydrochloric acid, is moreeffective as the acid solution in decreasing the pH since it contains ahigh hydrogen count.

Meanwhile, after the washing solution is made into an acid condition,the process of removing remaining cyanide 50 in which iron ions (herein,Fe²⁺) are separately input to the washing solution is performed. Duringthe process of removing remaining cyanide 50, as an amount of iron maybe increased in the washing solution, inverse reactions of those shownin Formulae (1) and (2) may be stimulated, thereby accelerating theprecipitation of the first cyanide.

As described above, through the processes of precipitating the firstcyanide 41, removing the second cyanide 42 and removing the remainingcyanide 50, the first cyanide is precipitated in the washing solution,and the second cyanide is dissolved into ammonia and carbon dioxide.

Afterwards, post-treatment 60 in which the washing solution in which thefirst cyanide is precipitated in a solid state is input to a knowncentrifugal separator to dissociate the solid from the liquid isperformed, thereby physically dissociating the first cyanide from thewashing solution.

Since all of the cyanide is removed from the washing solution, thephosphate concentration and pH are adjusted again by adding NaOH andphosphate, thereby being reused in the process of dissociating cyanide20.

Problems to be made in treatment of a contaminant such as nitrogen,phosphorus or TCE as well as cyanide, which is a subject of the presentinvention, are second contamination caused by using a treatment agentand a treatment cost. However, when the concentration and pH of thewashing solution are adjusted and recycled, these problems such as thesecond treatment and the high treatment cost may be solved.

In addition, the solid precipitate dissociated from the washing solutionincludes high-concentration first cyanide, so that it may be treatedwith a determined waste, or treated by applying high-temperatureoxidation suitable for treating solid cyanide having a low solubility.

The present inventor tested with treatment of cyanide obtained from thecyanide-contaminated soil.

To begin with, cyanide-contaminated soil was collected, and a content ofcyanide was measured by the test method for soil contamination widelyused in the art. Next, 30 mM of Na-pyrophosphate washing solution havinga pH of 12 was prepared, and reacted with the cyanide-contaminated soilin a weight ratio of 10 to 1 for 24 hours. Afterwards, the soil and thewashing solution were dissociated from each other using a centrifugalseparator, and the content of cyanide in the soil was measured by thetest method for soil contamination. The total cyanide content(concentration) in the washing solution was measured by the officialtest methods of water quality widely used in the art, and the content ofthe free cyanide was measured according to the ASTM. The result is shownin FIG. 5.

It can be confirmed that the cyanide-contaminated soil included cyanidehaving a concentration of 85.7 mg/kg before washing, but after beingwashed using the Na-pyrophosphate washing solution having a pH of 12,the concentration of the cyanide was deceased to 0.71 mg/kg, so that itcan be confirmed that almost all cyanide in the soil was dissociatedfrom the soil. It can be confirmed that the cyanide concentration in thesoil after the washing was far below the environment standard (“A” areaconcern standard: 2 mg/kg) according to the soil environmentconversation act, which indicates that the soil has no problem in beingreused to remediate.

Further, the total cyanide concentration in the washing solution was25.2 mg/L, and the free cyanide was insignificant, for example, 0.31mg/L. It is a natural result obtained by considering that the content ofthe free cyanide in the cyanide-contaminated soil is very small, and itcan be seen that most of the cyanide in the washing solution is SADcyanide.

Afterwards, an experiment including processes of precipitating firstcyanide 41, removing second cyanide 42 and removing remaining cyanide 50was continuously performed.

First, an oxidizing agent was input to inhibit generation ofhigh-toxicity HCN gas during the removal of the cyanide from the washingsolution and oxidize the WAD cyanide and the free cyanide. That is, H₂O₂was input to the washing solution until the concentration reached 0.4mg/L, and they reacted with each other for 10 minutes. The concentrationof the free cyanide was then measured by the ASTM.

Afterwards, H₂SO₄ was input to the washing solution to adjust pH of thewashing solution to 4, and they reacted with each other for 10 minutes.The total cyanide concentration in the washing solution was measured bythe official test methods of water quality.

Finally, to additionally provide iron ions in the washing solution,FeCl₂.4H₂O was input to adjust the Fe concentration in the washingsolution to 1 mg/L, and they reacted with each other for 10 minutesagain. Afterwards, the total cyanide concentration was measured by theofficial test methods of water quality. The result is shown in FIG. 6.

The free cyanide present at the concentration of 0.31 mg/L in thewashing solution was completely removed from the washing solution byreacting with H₂O₂, confirming that there is no chance to producehigh-toxicity HCN even when H₂SO₄ is input. The concentration of thecyanide in the washing solution was 25.2 mg/L, but after the washingsolution was oxidized by inputting H₂SO₄ to the washing solution,decreased to 1.9 mg/L, and a cyanide precipitant was formed in thewashing solution.

After Fe required to precipitate Fe—CN was additionally provided byinputting FeCb₂.4H₂O, the total cyanide concentration in the washingsolution was 0.27 mg/L, lower than the discharged water quality standard(1 mg/L). That is, almost all cyanide dissolved in the washing solutionwas precipitated or dissolved so as to be removed from the washingsolution. Accordingly, it was confirmed that the method according to thepresent invention can effectively remove the cyanide in the soil. Theexperiment described above was performed at room temperature andatmospheric pressure, and an additional external heat source was notrequired, unlike the high-temperature oxidation.

As described above, first, the process of dissociating cyanide from soilusing the washing solution was introduced to perform a subsequentprocess only to cyanide (washing solution), thereby reducinginefficiency shown in the conventional method of treating a contaminantin the contaminated soil throughout the process.

The method of remediating cyanide-contaminated soil according to thepresent invention is performed at room temperature, and thus is moreeconomical than the conventional high-temperature oxidation.

Further, cyanide is dissociated from soil, so that consumption of anoxidizing agent by an organic material, sulfide mineral, or manganeseoxide is solved. Thus, the cyanide can be economically and effectivelyremoved.

Since the present invention reuses a treatment solution used to treatcyanide, second contamination according to the cyanide treatment doesnot occur, which is environmentally friendly.

A method of remediating cyanide-contaminated soil according to thepresent invention can effectively dissociate cyanide from soil presentin a solid state, which is difficult to treat with conventional low-costroom-temperature oxidation, and can remove cyanide more economicallythan a conventional high-cost method because a process is performed atroom temperature.

First, a process of dissociating cyanide from soil using a washingsolution is introduced to perform a subsequent process only to cyanide(washing solution), thereby reducing inefficiency shown by theconventional method of treating a contaminant to the contaminated soilthroughout the process.

The method of remediating cyanide-contaminated soil according to thepresent invention is performed at room temperature, so that it is moreeconomical than the conventional high-temperature oxidation.

Further, cyanide is dissociated from soil, so that consumption of anoxidizing agent by an organic material, sulfide mineral, or manganeseoxide can be solved. Thus, the cyanide can be economically andeffectively removed.

Since the present invention reuses a treatment solution used to treatcyanide, second contamination according to the cyanide treatment doesnot occur, which is environmentally friendly.

While the invention has been shown and described with reference tocertain exemplary embodiments thereof, it will be understood by thoseskilled in the art that various changes in form and details may be madetherein without departing from the spirit and scope of the invention asdefined by the appended claims.

What is claimed is:
 1. A method of remediating cyanide-contaminatedsoil, the method comprising: collecting the soil contaminated withcyanide; mixing the soil with an alkali washing solution to dissolve thecyanide in the alkali washing solution; dissociating the soil away fromthe alkali washing solution; adding an oxidizing agent into the alkaliwashing solution to oxidize a soluble portion of the cyanide; andacidifying the alkali washing solution with an acid to precipitate afraction of the cyanide.
 2. The method according to claim 1, furthercomprising separating the precipitated fraction of the cyanide from theacidified washing solution.
 3. The method according to claim 1, whereinthe alkali washing solution has a pH of 9 to
 12. 4. The method accordingto claim 3, wherein the alkali washing solution has a pH of 11 to
 12. 5.The method according to claim 1, wherein the alkali washing solutionincludes a phosphate salt.
 6. The method according to claim 5, whereinthe phosphate salt in the alkali washing solution includes pyrophosphate(P₂O₇ ⁴⁻).
 7. The method according to claim 5, wherein the phosphatesalt in the alkali washing solution has a concentration of 30 to 50mM.8. The method according to claim 1, further comprising adding iron saltto the acidified washing solution to precipitate a residual part ofcyanide from the acidified washing solution.
 9. The method according toclaim 1, wherein acidifying the alkali washing solution is achieved byadding sulfuric acid (H₂SO₄) into the alkali washing solution.
 10. Themethod according to claim 2, wherein separating comprises centrifuging.11. The method according to claim 10, wherein the acidified washingsolution is reused by adding alkali into the acidified washing solutionto turn the acidified washing solution back into the alkali washingsolution.
 12. The method according to claim 1, wherein the oxidizingagent is selected from the group consisting of hydrogen peroxide (H₂O₂)and ozone (O₃).
 13. The method according to claim 1, wherein theoxidation of cyanide results in forming cyanate (OCN)⁻in the alkalinewashing solution.
 14. The method according to claim 13, wherein aportion of the cyanate is converted into ammonia (NH₃) and carbondioxide (CO₂) subsequent to acidifying the alkali washing solution. 15.The method according to claim 5, wherein the phosphate salt in thealkali washing solution is selected from the group consisting oforthophosphate (PO₄ ³⁻) salt, pyrophosphate (P₂O₇ ⁴⁻) salt, andhexametaphosphate (PO₃ ¹⁻)₆ salt.
 16. The method according to claim 1,wherein the method removes at least 99% of the cyanide from thecontaminated soil.
 17. The method according to claim 1, wherein the acidis selected from the group consisting of hydrochloric acid (HCI) andsulfuric acid (H₂SO₄).
 18. A method of remediating cyanide-contaminatedsoil, the method comprising: collecting the soil contaminated withcyanide; extracting the cyanide from the soil by mixing the soil with analkali washing solution to dissolve the cyanide into the alkali washingsolution, wherein the alkali washing solution comprises a pH between 10to 12 and a phosphate salt containing pyrophosphate (P₂O₇ ⁴⁻);separating the alkali washing solution from the soil; adding hydrogenperoxide (H₂O₂) into the separated alkali washing solution to oxidize aportion of cyanide in the separated alkali washing solution; acidifyingthe separated alkali washing solution with sulfuric acid to form anacidified washing solution and to precipitate cyanide; adding an ironsalt into the acidified washing solution to further precipitate cyanidefrom the acidified washing solution; separating the acidified washingsolution away from the precipitated cyanide.
 19. The method according toclaim 18, further comprising recycling the acidified washing solution byadding alkali into the acidified washing solution to turn the acidifiedwashing solution back into the alkali washing solution.
 20. A method ofremediating cyanide-contaminated soil, the method comprising: collectingthe soil contaminated with cyanide; extracting the cyanide from the soilby mixing the soil with an alkali washing solution to dissolve thecyanide into the alkali washing solution, wherein the alkali washingsolution comprises a pH between 10 to 12 and a phosphate salt;separating the alkali washing solution from the soil; adding anoxidizing agent into the separated alkali washing solution to oxidize aportion of cyanide in the separated alkali washing solution; acidifyingthe separated alkali washing solution with an acid to form an acidifiedwashing solution and to precipitate cyanide; adding an iron salt intothe acidified washing solution to further precipitate cyanide from theacidified washing solution; separating the acidified washing solutionaway from the precipitated cyanide; and recycling the acidified washingsolution by adding alkali into the acidified washing solution to turnthe acidified washing solution back into the alkali washing solution.